Referring to FIG. 1, certain mobile communications networks include relay nodes (R) that operate in cooperative fashion to decode and forward data communicated from a source node (S) to a destination node (D). A mobile terminal (S) thus may be connected to the base station (D) directly or through a relay node (R). If a mobile terminal has no relays around it, the mobile terminal is connected to the base station directly. However, if a mobile terminal is in the proximity of a relay node and if using the relay node is better in terms of certain metrics, such as energy consumption, etc. the terminal may be connected to the base station through the relay node.
In wireless networks, where mobile terminals communicate via relay nodes, signal to noise ratio (SNR) of those links in cooperative transmission frequently generate imbalance in transmit power. For example, suppose that there are one source node, one relay node, and one destination node. Given that the transmit power levels are unbalanced, one node consumes more power than it eventually contributes to the throughput that the destination node may achieve, whereas the other node causes a lower-limit in the throughput.
As shown in FIG. 1, in a decode and forward scenario, series of time frames are utilized, where each time frame has two time slots. In the first time slot, the source node transmits at power level P1, and the relay and destination nodes receive the signal from the source node. In the second time slot, the relay forwards the information received from the source node to the destination node and the destination node combines the two signals, one directly from the source node and the other relayed by the relay node, to take advantage of cooperation diversity among the nodes.
Communication through relay nodes as studied in the context of cellular networks has the following benefits: (1) it significantly reduces a transmit power requirement for the same data rate compared to those for a base station, thus permitting economical amplifier design in the relay; (2) it eliminates the costs associated with building a wired infrastructure that serves as the interface between the base station and backend systems; and (3) it significantly increases a data transmission rate for the same power compared to those for a base station, thus potentially solving the coverage problem for high data rates in larger cells.
In a cooperative relay network, it is assumed that information available at a certain relay may be shared among a subset of relays or all relays. Two broad categories of cooperative relaying techniques are cooperative diversity (C-DIV) and cooperative spatial multiplexing (C-SM). In C-DIV each relay may be considered as an antenna element in distributed antenna array. Cooperation at the relays is done in such a way to provide increased reliability of reception, such as bit error rate, through the diversity gain at the destination. A typical example is space time block code (STBC) encoding at the relays, e.g., Alamouti encoding.
In C-SM each relay is responsible to detect a subset of transmitted information (data stream) from the source. For example, consider that the source transmits a 16 QAM signal and 4 relays are in the system. Each relay may detect one bit information out of the incoming 16 QAM signal and transmits extracted one bit information using a BPSK signal. All relays forward their low-rate sub-streams simultaneously to the destination over the same physical channel. It may reduce the required transmission power by reducing the transmission rate per relay while maintaining the same Eb/N0 requirement. Reduced transmission power provides less interference, thus increased capacity.
In C-DIV each relay needs to process and transmit at the same data rates as the one the source generates. Unfortunately, for high-rate data, hardware complexity and energy consumption on each relay may be substantial. Furthermore, incorrect estimation of the transmitted signal at the relays propagates to the destination, thus diversity gain may not be achieved. Also, in C-SM further enhancement in terms of bit error rate (BER) or frame error rate (FER) are needed when some sort of transmit diversity is incorporated. System architectures for cooperative relay-based communication systems are needed that may support both C-DIV and C-SM.